1. Field of the Invention
The present invention relates to an optical device which is mounted in a photographing optical system such as a optical system of video camera or still camera to particularly prevent ghost or flare in an photographed image.
2. Description of the Related Art
A photographing optical system such as a video camera or still camera uses an aperture device which changes the aperture size by using a plurality of aperture blades to adjust the quantity of light. If the aperture size is excessively small, diffraction of light poses a problem of degradation of the image quality. In view of this, in order to prevent the aperture size from becoming excessively small even when photographing a high-luminance object, an aperture device which employs an ND (Neutral Density) filter together with aperture blades has been proposed.
An aperture edge 1a of an aperture blade 1 which forms an aperture may not receive any machining or processing at all after cutting, as shown in the partially enlarged view of FIG. 16. In this case, when light strikes the aperture edge 1a, reflected light beams are not scattered in random direction. They intensify each other on an image forming surface in a certain direction with respect to the surface of the aperture edge 1a. If this unwanted reflection occurs in spaces among a plurality of photographing lenses, flare or ghost occurs in the image and degradation of the image quality occurs.
In view of this, a black coating is applied to the inner side of a lens barrel or the surfaces of the aperture blades to suppress reflection, or the ND filter is mounted in the barrel to suppress ghost or the like. When photographing a particularly high-luminance object such as the sun or an intense light source, however, a radial ghost may occur around the object image. This is because the light that has struck a portion near the ridgeline of the edge portion of the aperture blade or the end face of the edge portion of the ND filter forms harmful light.
As a prior art, e.g., Japanese Patent Laid-Open No. 5-281590 discloses a known technique in which small steps are formed on the end face of the aperture edge of the aperture blade entirely or partly. The steps scatter the harmful light generated by reflection at the end surface of the aperture blade, to the left and right with respect to the incident direction of light to prevent concentration of the harmful light.
Japanese Patent Laid-Open No. 2002-229095 also discloses a known technique in which a plurality of steps in a stair form on the end face of an aperture blade are irregularly formed. The steps in the stair form scatter the harmful light generated by reflection at the end face of the aperture blade, back and forth with respect to the incident direction of light to prevent concentration of the harmful light.
Japanese Patent Laid-Open No. 2-190833 discloses an aperture device in which an ND filter having plural regions with various uniform transmittances is attached to shade the aperture that the aperture blades form. Plural regions of the ND filter are formed such that their transmittances decrease gradually toward the aperture edge. This prevents degradation in image quality which occurs due to the small aperture size.
The surface of the image sensor or the surface of a cover glass, crystal low-pass filter, an infrared cut filter, or the like may reflect part of the light, which has been transmitted through the ND filter to enter the image sensor, and reflected light may return to the object side. The ND filter surface facing to the image sensor or the ND filter surface facing to the object reflects the return light again. And reflected light forms ghost light that enters the image sensor. To decrease the ghost light, in the aperture device disclosed in Japanese Patent Laid-Open No. 2000-36917, an anti-reflection film is formed on the surface of the ND filter facing to the image sensor.
An ND filter is available which is obtained by a method of mixing and kneading a light absorbing organic dyes or pigments in a plastic material. Also, an ND filter is known which is obtained by forming a light absorbing film on the surface of a plastic substrate in accordance with the evaporation method. The advantage of the former filter resides in that it enables manufacturing of a large amount of filters having a uniform density at a low cost. In the former filter, however, the transmittance changes more than in the latter filter with respect to a wavelength within the wavelength range of visible light, leading to a larger non-uniformity. Therefore, as the ND filter to be used in the aperture device, those manufactured by the evaporation method are preferably employed.
As the plastic substrate of the ND filter manufactured by the evaporation method, a film-like plastic substrate which has a small haze factor representing a haze and has a large light transmittance is employed. More specifically, for example, ND filter is known which is obtained by forming a light absorbing film on a transparent plastic substrate made of polyethylene terephthalate, polycarbonate, acrylate, cycloolefin, polymethylpentene, or the like.
If the plastic substrate has a large haze factor, the image quality is degraded because of scattering of light. A plastic substrate is preferably employed to manufacture a ND filter because it is lightweight and can be cut readily. The ND filter employing the plastic substrate has a thickness of 50 μm to 300 μm, and preferably 100 μm.
The substrate of the ND filter is a plastic substrate. As shown in FIG. 17, if an edge portion 2a of an ND filter 2 does not receive particular machining or processing, the edge portion 2a reflects some part of light that has hit a end face or a portion near the ridgeline of the edge portion 2a. And the other part (not shown) of the light is transmitted through the edge portion 2a. Thus, the reflected or transmitted light does not form scattered light that diffuse in random directions, but tends to form ghost light which is uniformly reflected by the edge portion 2a or ghost light which has uniformly transmitted through the edge portion 2a. As a result, on the image forming surface, ghost occurs in the vertical direction with respect to the edge portion of the ND filter. The lower the density of the ND filter is, the more remarkable the ghost becomes.
With the solution disclosed in Japanese Patent Laid-Open No. 5-281590 of forming uneven portions on the end face of the aperture blade, the uneven portions merely distribute the ghost light to the left and right with respect to the propagating direction. Thus, the ghost or the like does decrease but not sufficiently.
With the solution of forming a stair form portion on the end face of the aperture blade from the entering side of aperture to the exit side of it, as in Japanese Patent Laid-Open No. 2002-229095, the portion near the exit side ridgeline of the edge portion of the aperture blade is the same as in the prior art. Thus, reflection at this portion generates ghost light or the like.
Even when forming such a stair form portion on the ND filter, light does exist which is transmitted through the plastic substrate and travels toward the exit side, as shown in FIG. 18. Thus, formation of the stair form portion does not solve the problem.
Each of FIGS. 19A to 19C shows the shape of an edge portion 6a of a conventional ND filter 6 which is inserted to shade an aperture formed by aperture blades and to adjust the quantity of entering light. The ridgeline of the edge portion 6a is formed with an angle to suppress ghost light or the like. However, when the object has a high illuminance, mere forming the ridgeline portion having an angle cannot sufficiently eliminate ghost or flare. Hence, reflected light at the end face and transmitted light entering from the end face are not negligible.